Specifications for developing Diesel engines are constantly changing. This technical constraint is mainly linked to the fields of environment and economy such as the emissions pollutants (nitrogen oxides, hydrocarbons, particles, etc.), the noise made by the engine or fuel consumption. The requirements in terms of optimizing the combustion environment to take into account these evolutions in specifications require a particular effort regarding the injection process. The ideal injection which would make it possible to obtain a pollution-free combustion would be achieved if:
1. the start of fuel inlet is performed at low flow rate so as to not mix too much fuel with the air from the combustion chamber during the ignition time, the injected flow constantly increases so that the combustion fully accompanies the start of expansion associated with movement of the piston in the engine's cylinder, PA0 2. the fuel pressure is important to obtain proper pulverization and consequently good mixing of the fuel with the air, PA0 3. the end of the injection is clear-cut to limit the insufficiently pulverized fuel inlet and reduce combustion trails as much as possible. PA0 increasing the compression ratio, PA0 reducing the injection advance, PA0 increasing injection pressures. PA0 a more moderate injection start than with conventional devices, favorable to item 1 mentioned above, with the possibility of performing a pre-injection, PA0 an injection pressure which increases during the whole injection period, favorable to item 2 above, PA0 an end of injection which is as clear-cut as with constant pressure devices, favorable to item 4 above, PA0 an adjustable injection advance.
In practice, conventional strategies generally used are for example
These strategies aim to compress the main combustion period into a shorter period of time which is better located at the start of the pressure reduction. Despite everything, the combustion performance remains very sensitive to the details of form of the law on fuel inlet in the combustion chamber.
In standard injection devices using a pulsating pump, the injection pump, by delivering the fuel, makes the pressure increase progressively in the pump's volumes, the conduits and the injector. This progressive increase takes place before and then during the injection period. After the pump has stopped delivering, the injection ends with the effect of the depressurization of these same volumes, the injector needle being solely controlled by a basic return device comprising one or several springs.
The advantage of these injection devices relates to the injection start which, in this case, is relatively moderate and, consequently, favorable to items 1 and 2 mentioned above, unless one needs too high a cutting-in pressure for the injector.
On the other hand, the major drawback is that the injector only closes when the pressure has become much lower than the cutting-in pressure. As a result, the end of the injection is not efficient and generates combustion trails, bringing about emissions of soot and penalizing efficiency.
In so-called "Common-Rail" constant pressure injection devices, the high-pressure pump feeds all the injectors at a virtually constant and adjustable pressure to adjust the inlet rate and the pulverization of fuel. The opening and the closing of each injector are controlled by one electrovalve, which makes it possible to adjust the injection advance and the quantity injected, in accordance with certain examples of embodiment described in publications FR-A-2 016 477, U.S. Pat. No. 4,545,352, DE-C-42 36 882, DE-A-44 06 901 and U.S. Pat. No. 4,249,497.
The advantage of these injection devices is the flexibility of the potential adjustments and especially the very good end of injection by controlled closing, which is favorable to items 3 and 4 above.
Nevertheless, the major drawback lies in the fact that at the start of the injection, the injected flow very quickly reaches the maximum flow, which is detrimental to items 1 and 2 above. It is possible to neutralize the effect on the ignition deflagration (item 1) by using the pre-injection, but there is little chance of making the law of fuel inlet (item 2) progressive.